struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2){
	//递归条件
    if(l1 == NULL) return l2;
    if(l2 == NULL) return l1;

    if(l1->val < l2->val){
        l1->next = mergeTwoLists(l1->next,l2);
        return l1;
    }
    else{
        l2->next = mergeTwoLists(l1,l2->next);
        return l2;
    }
}
